An electric power steering apparatus (EPS) which provides a steering mechanism of a vehicle with a steering assist torque (an assist torque) by means of a rotational torque of a motor, applies a driving force of the motor as an actuator to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the steering assist torque, such a conventional electric power steering apparatus performs a feed-back control of a motor current. The feed-back control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage supplied to the motor is generally performed by an adjustment of duty command values of a pulse width modulation (PWM) control.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft or a handle shaft) 2 connected to a handle 1 is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a pinion-and-rack mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. In addition, the steering shaft 2 is provided with a torque sensor 10 for detecting a steering torque Th of the handle 1, and a motor 20 for assisting the steering torque of the handle 1 is connected to the column shaft 2 through the reduction gears 3. The electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist command (a steering assist command) on the basis of the steering torque Th detected by the torque sensor 10 and a vehicle speed Vs detected by a vehicle speed sensor 12, and controls a current supplied to the motor by means of a voltage control command value Vref obtained by performing compensation or the like to the calculated current command value. A steering angle sensor 14 is not indispensable and may not be provided. It is possible to obtain the steering angle (a motor rotational angle) θ from a rotational position sensor such as a resolver which is connected to the motor 20.
The controller area network (CAN) 40 to send/receive various information and signals on the vehicle is connected to the control unit 30, and it is also possible to receive the vehicle speed Vs from the CAN. Further, a Non-CAN 41 is also possible to connect to the control unit 30, and the Non-CAN 41 sends and receives a communication, analogue/digital signals, electric wave or the like except for the CAN 40.
In such an electric power steering apparatus, the control unit 30 mainly comprises a central processing unit (CPU) (including a micro processing unit (MPU) and a micro controller unit (MCU)), and general functions performed by programs within the CPU are, for example, shown in FIG. 2.
Functions and operations of the control unit 30 will be described with reference to FIG. 2. The steering torque Th from the torque sensor 10 and the vehicle speed Vs from the vehicle speed sensor 12 are inputted into a current command value calculating section 31. The current command value calculating section 31 calculates a current command value Iref1 based on the steering torque Th and the vehicle speed Vs using an assist map or the like. The calculated current command value Iref1 is added with a compensation signal CM for improving characteristics from a compensating section 34 at an adding section 32A. The current command value Iref2 after addition is limited of the maximum value thereof at a current limiting section 33. The current command value Irefm limited of the maximum value is inputted into a subtracting section 32B, whereat a detected motor current value Im is subtracted from the current command value Irefm.
The subtraction result ΔI (=Irefm−Im) in the subtracting section 32B is current-controlled such as a proportional integral (PI) at a at a PI-control section 35. The voltage control value Vref obtained by the current-control, and a modulation signal (a carrier) CF are inputted into a PWM-control section 36, whereat a duty thereof is calculated. The motor 20 is PWM-driven by an inverter 37 with a PWM-signal calculated the duty. The motor current value Im of the motor 20 is detected by a motor current detection means 38 and is inputted into the subtracting section 32B for the feedback.
The compensating section 34 adds a self-aligning torque (SAT) detected or estimated and an inertia compensation value 342 at an adding section 344. The addition result is further added with a convergence control value 341 at an adding section 345. The addition result is inputted into the adding section 32A as the compensation signal CM, thereby to improve the control characteristics.
Recently, the three-phase brushless motor is mainly used to the actuator of the electric power steering apparatus. Since the electric power steering apparatus is an on-vehicle product, the inverter, which drives the motor, in comparison with general industries such as home electric appliances, needs to have a large dead time (“industrial equipment”<“EPS”) in view of a wide operating temperature range and a fail-safe. Generally, since a switching device (for example, a field-effect transistor (an FET)) has a delay time when turning-OFF, when an upper arm and a lower arm of the switching devices are turned-ON or turned-OFF at the same time, a situation that a direct current (DC) link is short circuit is occurred. In order to prevent from the above case, the time (the dead time), which both the upper and lower arms of the switching devices turn-OFF, is set.
As a result, the current waveform is distorted, and the response of the current control and a steering feeling are badly affected. For example, when the driver slowly steers the handle in a situation that the handle is around a straight running state (an on-center state), a discontinuous steering feeling by means of the torque ripple and like is occurred. Further, since the back-EMF of the motor at a while speed steering maneuver or a high speed steering maneuver and the interference voltage between the windings act as the disturbance against the current-control, a steering follow-up performance and a steering feeling at a turning back maneuver go down.
A q-axis to control the torque being the coordinate axis of a rotor of the 3-phase brushless motor and a d-axis to control the magnetic field strength are independently set and has a relation that the d-axis and the q-axis is 90°. Thus, a vector control system to control the currents (the d-axis current command value and the q-axis current command value) corresponding to respective axes with the vector, is known.
FIG. 3 shows a configuration example in a case that a 3-phase brushless motor 100 is driving-controlled by the vector control system. A d-axis current command value id* and a q-axis current command value iq*, which are calculated in the current command value calculating section (not shown) based on the steering torque Th, the vehicle speed Vs and so on, of a dq-axis coordinate system of two axes are respectively inputted into subtracting sections 131d and 131q, and current deviations Δid* and Δiq* obtained in the subtracting sections 131d and 131q are respectively inputted into PI-control sections 120d and 120q. Voltage command values vd and vq PI-controlled in the PI-control sections 120d and 120q are respectively inputted into a subtracting section 121d and an adding section 121q, and voltages Δvd and Δvq obtained in the subtracting section 121d and the adding section 121q are inputted into a dq-axis/3-phase alternative current (AC) converting section 150. Voltage command values Vu*, Vv*, Vw* converted into 3-phases in the dq-axes/3-phase AC converting section 150 are inputted into a PWM-control section 160, and the motor 100 is driven with calculated duties via the inverter 161.
The 3-phase motor currents iu, id, iw of the motor 100 are detected by current detectors 162, and the detected 3-phase motor currents iu, id, iw are inputted into a 3-phase AC/dq-axes converting section 130. Feedback currents id and iq of 2-phases converted in the 3-phase AC/dq-axes converting section 130 are respectively inputted into subtracting sections 131d and 131q, and further inputted into a d-q non-interference control section 140. Further, a rotational sensor or the like is attached to the motor 100, and a motor rotational angle ω and a motor rotational speed (a rotational velocity) ω are outputted from an angle detecting section 110 to process the sensor signal. The motor rotational angle θ is inputted into the dq-axes/3-phase AC converting section 150 and the 3-phase AC/dq-axes converting section 130, and the motor rotational speed ω is inputted into the d-q non-interference control section 140.
Such an electric power steering apparatus of the vector control method is an apparatus that the steering of the driver is assisted, and the noisy sound, the vibration of the motor, the ripple and the like are transmitted as the force feeling to the driver via the handle. Further, the inverter sets the dead time so that the switching devices of the upper arm and the lower arm are not short-circuited. Since this dead time is non-linear, the current waveform is distorted, a responsibility of the control goes down and the noisy sound, the vibration the ripple and so on are occurred. Since an arrangement of the motor, which is directly coupled to a gearbox connected to the handle and the steel column shaft in a case of a column type electric power steering apparatus, is extremely close to the driver in considering the structure, the noisy sound, the vibration, the ripple and the like that are caused by the motor need to especially be considered in comparison with a downstream assist type electric power steering apparatus.
FIG. 4 shows a result in a case that sinusoidal wave is inputted into the d-axis current command value (a reference value) in the general dq-axis vector control (FIG. 3). It is understood that the waveform of the current measuring value is distorted for the d-axis current command value. Showing the motor current when the handle is slowly steered from the straight running state (the on-center state), as shown in FIG. 5 and FIG. 6, it is understood that the vibration and the ripple of the q-axis current (the torque) are large due to the distortion of the phase currents. FIG. 5 shows the U-phase motor current, the V-phase motor current and the W-phase motor current for the d-axis current command value and the q-axis current command value, and FIG. 6 shows only the q-axis current command value and the U-phase motor current from FIG. 5.
Conventionally, as a method to compensate the dead time of the inverter, there are methods to add the compensation value by detecting a timing occurring the dead time and to compensate the dead time by a disturbance observer on the dq-axes in the current control.
In the control unit of the electric power steering apparatus disclosed in Japanese Patent No. 3706296 B2 (Patent Document 1), the disturbance voltage estimating observer, which measures the disturbance voltage generated in the motor by using the voltage applied to the motor and the present current value of the motor and outputs a signal corresponding to the disturbance voltage, is disposed, and the dead time of the inverter is compensated. Further, in the control unit of the voltage type inverter disclosed in Japanese Unexamined Patent Publication No. 2007-252163 A (Patent Document 2), the disturbance estimating observer, which estimates the disturbance voltage including an output voltage error which is caused by the dead time of the inverter and an back-EMF electric power component of the motor, is disposed, and the dead time of the inverter is compensated.